Imaging apparatus including unit pixel, counter electrode, photoelectric conversion layer, and computing circuit

ABSTRACT

An imaging apparatus includes a unit pixel including a pixel electrode; a counter electrode facing the pixel electrode; a photoelectric conversion layer disposed between the pixel electrode and the counter electrode; and a computing circuit that acquires a first signal upon a first voltage being applied between the pixel electrode and the counter electrode, the first signal corresponding to an image captured with visible light and infrared light and a second signal upon a second voltage being applied between the pixel electrode and the counter electrode, the second signal corresponding to an image captured with visible light, and generates a third signal by performing a computation using the first signal and the second signal, the third signal corresponding to an image captured with infrared light.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/595,292, filed on Oct. 7, 2019, which is a Continuation of U.S.patent application Ser. No. 15/873,190, filed on Jan. 17, 2018, now U.S.Pat. No. 10,477,121, which in turn claims the benefit of JapaneseApplication No. 2017-018776, filed on Feb. 3, 2017, the entiredisclosures of which Applications are incorporated by reference herein.

BACKGROUND 1. Technical Field

The present disclosure relates to an imaging apparatus.

2. Description of the Related Art

When a color image based on visible light and a monochrome image basedon infrared light are captured with one image sensor in the environmentin which the intensity of infrared light is high, infrared lightdegrades color reproductivity. Accordingly, there has been devised atechnique in which an infrared cutoff filter capable of blockinginfrared light is disposed on the front surface of the sensor prior tothe capturing of a color image in order to achieve good colorreproductivity (e.g., see Japanese Unexamined Patent ApplicationPublication No. 2010-109875). The infrared cutoff filter may be removed,for example, at nighttime in order to capture a monochrome image. In thecase where the ambient light includes both infrared light and visiblelight, using a visible-light cutoff filter capable of blocking visiblelight may enable the capturing of an image based on only infrared light.

SUMMARY

In one general aspect, the techniques disclosed here feature an imagingapparatus including a unit pixel including a pixel electrode, a chargeaccumulation region electrically connected to the pixel electrode, and asignal detection circuit electrically connected to the chargeaccumulation region; a counter electrode facing the pixel electrode; aphotoelectric conversion layer disposed between the pixel electrode andthe counter electrode; and a computing circuit. The photoelectricconversion layer converts visible light and infrared light into a firstelectrical signal upon a first voltage being applied between the pixelelectrode and the counter electrode. The photoelectric conversion layerconverts the visible light into a second electrical signal upon a secondvoltage being applied between the pixel electrode and the counterelectrode, the second voltage being different from the first voltage.The computing circuit acquires a first signal on a basis of the firstelectrical signal upon the first voltage being applied between the pixelelectrode and the counter electrode, the first signal corresponding toan image captured with the visible light and the infrared light. Thecomputing circuit acquires a second signal on a basis of the secondelectrical signal upon the second voltage being applied between thepixel electrode and the counter electrode, the second signalcorresponding to an image captured with the visible light. The computingcircuit generates a third signal by performing a computation using thefirst signal and the second signal, the third signal corresponding to animage captured with the infrared light.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary circuitstructure of an imaging apparatus according to an embodiment;

FIG. 2 is a schematic cross-sectional view of a unit pixel according toan embodiment, illustrating an exemplary device structure of the unitpixel;

FIG. 3 is a schematic cross-sectional view of a photoelectric conversionlayer according to an embodiment, illustrating an example structure ofthe photoelectric conversion layer;

FIG. 4 includes timing charts used for explaining an example of theaction of an imaging apparatus according to an embodiment;

FIG. 5 is a diagram illustrating an example of the voltage dependence ofthe spectral sensitivity characteristic of a photoelectric conversionlayer included in an imaging apparatus according to an embodiment;

FIG. 6A is a diagram illustrating an example of the spectral sensitivitycharacteristic of a photoelectric conversion layer according to anembodiment with respect to visible light and infrared light;

FIG. 6B is a diagram illustrating an example of the spectral sensitivitycharacteristic of a photoelectric conversion layer according to anembodiment with respect to visible light;

FIG. 6C is a diagram illustrating an example of the spectral sensitivitycharacteristic of a photoelectric conversion layer according to anembodiment with respect to infrared light;

FIG. 6D is a schematic diagram illustrating the structure of the pixelaccording to an embodiment and signals acquired by the pixel in avisible-light-and-infrared mode;

FIG. 6E is a schematic diagram illustrating the structure of the pixelaccording to an embodiment and signals acquired by the pixel in avisible-light mode;

FIG. 6F is a schematic diagram illustrating the structure of the pixelaccording to an embodiment and signals acquired by the pixel in aninfrared mode;

FIG. 7 includes diagrams illustrating an example of a switchover betweenmodes performed by an imaging apparatus according to an embodiment; and

FIG. 8 includes diagrams illustrating another example of a switchoverbetween modes performed by an imaging apparatus according to anembodiment.

DETAILED DESCRIPTION

When the imaging method described in Description of the Related Artabove, in which a switchover between color imaging and infrared imagingis performed by mechanically attaching or detaching a movable filter, isused, it is not possible to capture good images while the filter isattached to or detached from the sensor. Therefore, the above imagingmethod is useful in applications where the switchover between colorimaging and infrared monochrome imaging is performed at a low frequencyas in day-night cameras. However, in the case where a color image and aninfrared image are simultaneously captured by the imaging method, themotion of the subject which is made while the filter is attached to ordetached from the sensor causes motion blur. Furthermore, it is notpreferable to move the movable part repeatedly at a high speed inconsideration of the durability and power consumption of the imagingapparatus.

An imaging apparatus according to an embodiment of the presentdisclosure includes a unit pixel including a pixel electrode, a chargeaccumulation region electrically connected to the pixel electrode, and asignal detection circuit electrically connected to the chargeaccumulation region; a counter electrode facing the pixel electrode; aphotoelectric conversion layer disposed between the pixel electrode andthe counter electrode; and a computing circuit.

The photoelectric conversion layer converts visible light and infraredlight into a first electrical signal upon a first voltage being appliedbetween the pixel electrode and the counter electrode.

The photoelectric conversion layer converts the visible light into asecond electrical signal upon a second voltage being applied between thepixel electrode and the counter electrode, the second voltage beingdifferent from the first voltage.

The computing circuit acquires a first signal on a basis of the firstelectrical signal upon the first voltage being applied between the pixelelectrode and the counter electrode, the first signal corresponding toan image captured with the visible light and the infrared light.

The computing circuit acquires a second signal on a basis of the secondelectrical signal upon the second voltage being applied between thepixel electrode and the counter electrode, the second signalcorresponding to an image captured with the visible light.

The computing circuit generates a third signal by performing acomputation using the first signal and the second signal, the thirdsignal corresponding to an image captured with the infrared light.

The above-described imaging apparatus is capable of performing aswitchover between imaging using visible light and imaging using visiblelight and infrared light simultaneously for all pixels by changing thevoltage applied to the photoelectric conversion layer. The imagingapparatus is also capable of generating an image captured using infraredlight from an image captured using visible light and an image capturedusing visible light and infrared light. Thus, the imaging apparatus iscapable of capturing an image based on visible light, an image based onvisible light and infrared light, and an image based on infrared lightwithout using a movable filter.

The imaging apparatus according to an embodiment of the presentdisclosure may further include a voltage supply circuit configured toselectively apply the first voltage or the second voltage between thepixel electrode and the counter electrode.

In the imaging apparatus according to an embodiment of the presentdisclosure, the computing circuit may generate the third signal bysubtracting, from the first signal, a signal obtained by multiplying thesecond signal by a gain.

In the imaging apparatus according to an embodiment of the presentdisclosure, the gain may correspond to the ratio of a quantum efficiencyof the photoelectric conversion layer which is achieved upon the firstvoltage being applied between the pixel electrode and the counterelectrode to a quantum efficiency of the photoelectric conversion layerwhich is achieved upon the second voltage being applied between thepixel electrode and the counter electrode.

In the imaging apparatus according to an embodiment of the presentdisclosure, the gain may correspond to the product of

the ratio of a quantum efficiency of the photoelectric conversion layerwhich is achieved upon the first voltage being applied between the pixelelectrode and the counter electrode to a quantum efficiency of thephotoelectric conversion layer which is achieved upon the second voltagebeing applied between the pixel electrode and the counter electrode, and

the ratio of a first exposure time to a second exposure time, the firstexposure time being a time during which the first voltage is appliedbetween the pixel electrode and the counter electrode, the secondexposure time being a time during which the second voltage is appliedbetween the pixel electrode and the counter electrode.

In the imaging apparatus according to an embodiment of the presentdisclosure, the computing circuit may generate the third signal bysubtracting a fifth signal from a fourth signal, the fourth signal beingobtained by multiplying the first signal by a first gain, the fifthsignal being obtained by multiplying the second signal by a second gain,the second gain being different from the first gain.

In the imaging apparatus according to an embodiment of the presentdisclosure, the ratio of the first gain to the second gain maycorrespond to the ratio of a quantum efficiency of the photoelectricconversion layer which is achieved upon the first voltage being appliedbetween the pixel electrode and the counter electrode to a quantumefficiency of the photoelectric conversion layer which is achieved uponthe second voltage being applied between the pixel electrode and thecounter electrode.

In the imaging apparatus according to an embodiment of the presentdisclosure, the ratio of the first gain to the second gain maycorrespond to the product of

the ratio of a quantum efficiency of the photoelectric conversion layerwhich is achieved upon the first voltage being applied between the pixelelectrode and the counter electrode to a quantum efficiency of thephotoelectric conversion layer which is achieved upon the second voltagebeing applied between the pixel electrode and the counter electrode, and

the ratio of a first exposure time to a second exposure time, the firstexposure time being a time during which the first voltage is appliedbetween the pixel electrode and the counter electrode, the secondexposure time being a time during which the second voltage is appliedbetween the pixel electrode and the counter electrode.

In the imaging apparatus according to an embodiment of the presentdisclosure, the first exposure time may be shorter than the secondexposure time.

In the imaging apparatus according to an embodiment of the presentdisclosure, the photoelectric conversion layer may include photoelectricconversion films.

In the imaging apparatus according to an embodiment of the presentdisclosure, at least one of the photoelectric conversion films mayinclude an organic material.

In the present disclosure, all or a part of a circuit, a unit, a device,a member, or a portion, or a part or all of functional blocks in theblock diagrams may be implemented as one or more of electronic circuitsincluding, but not limited to, a semiconductor device, a semiconductorintegrated circuit (IC) or a large scale integration (LSI). The LSI orIC may be integrated into one chip, or may alternatively be acombination of plural chips. For example, functional blocks other than amemory may be integrated into one chip. Although an LSI and an IC arereferred to above, the terms for the circuits may vary depending on thedegree of integration; they may also be referred to as system LSI, verylarge scale integration (VLSI), or ultra large scale integration (ULSI).A field programmable gate array (FPGA) that can be programmed aftermanufacturing an LSI or a reconfigurable logic device that allowsreconfiguration of the connection or setup of circuit cells inside theLSI can be used for the same purpose.

Furthermore, all or a part of the functions or operations of a circuit,a unit, a device, a member, or a portion may be implemented by executingsoftware. In such a case, the software is recorded on one or morenon-transitory recording media such as a ROM, an optical disk or a harddisk drive, and when the software is executed by a processor, thesoftware causes the processor together with peripheral devices toexecute the functions specified in the software. A system or apparatusmay include such one or more non-transitory recording media on which thesoftware is recorded and a processor together with necessary hardwaredevices such as an interface.

Embodiments of the present disclosure are described below in detail withreference to the attached drawings. In the following embodiments,general or specific examples are described. All the values, shapes,materials, components, the arrangement of the components, and theconnection between the components, steps, the order of the steps, andthe like described in the following embodiments are merely an exampleand are not intended to limit the scope of the present disclosure. Thevarious aspects described herein may be combined with one another unlessa contradiction arises. Among the components described in the followingembodiments, components that are not described in any one of theindependent claims, which indicate the broadest concepts, are describedas optional components. In the following description, components thathave substantially the same function are denoted by the same referencenumeral, and the description thereof may be omitted.

Imaging Apparatus

FIG. 1 is an exemplary circuit structure of an imaging apparatusaccording to an embodiment of the present disclosure. The imagingapparatus 100 illustrated in FIG. 1 includes a pixel array 200 thatincludes a plurality of unit pixels 10 arranged in a two-dimensionalarray. FIG. 1 schematically illustrates an example where the unit pixels10 are arranged in a matrix having two rows and two columns. The numberand arrangement of the unit pixels 10 included in the imaging apparatus100 are not limited to those in the example illustrated in FIG. 1.

The unit pixels 10 each include a photoelectric conversion unit 13 and asignal detection circuit 14. As described below with reference to thedrawings, the photoelectric conversion unit 13 includes two electrodesfacing each other and a photoelectric conversion layer interposedtherebetween and generates signal charge carriers upon receivingincident light. The photoelectric conversion unit 13 is not necessarilyan element in which all the components are exclusively provided for eachof the unit pixels 10. For example, some or all of the components of thephotoelectric conversion unit 13 may extend across the plurality of unitpixels 10. In this embodiment, a portion or the entirety of theincident-light-side electrode extends across the plurality of unitpixels 10.

The signal detection circuit 14 is a circuit that detects the signalgenerated by the photoelectric conversion unit 13. In this example, thesignal detection circuit 14 includes a signal detection transistor 24and an address transistor 26. The signal detection transistor 24 and theaddress transistor 26 are typically field-effect transistors (FETs). Inthis example, the signal detection transistor 24 and the addresstransistor 26 are N-channel metal oxide semiconductor field effecttransistors (MOSFETs).

As described schematically in FIG. 1, the control terminal (i.e., thegate) of the signal detection transistor 24 is electrically connected tothe photoelectric conversion unit 13. The signal charge carriers (i.e.,holes or electrons) generated by the photoelectric conversion unit 13are accumulated at a charge accumulation node 41 (also referred to as“floating diffusion node”), which is located between the gate of thesignal detection transistor 24 and the photoelectric conversion unit 13.The charge accumulation node 41 corresponds to a charge accumulationregion. The detailed structure of the photoelectric conversion unit 13is described below.

The photoelectric conversion unit 13 included in each unit pixel 10 isfurther connected to the corresponding one of sensitivity control lines42. In the example structure illustrated in FIG. 1, the sensitivitycontrol lines 42 are connected to a voltage supply circuit 32. Thevoltage supply circuit 32 is a circuit capable of selectively applyingany one of at least three types of voltages, that is, a first voltage, asecond voltage, and a third voltage, to the photoelectric conversionunits 13. While the imaging apparatus 100 is operated, the voltagesupply circuit 32 applies a predetermined voltage to the photoelectricconversion units 13 through the sensitivity control lines 42. Thevoltage supply circuit 32 is not limited to a specific power sourcecircuit. The voltage supply circuit 32 may be a circuit that generatesthe predetermined voltage or a circuit that converts a voltage appliedby another power source into the predetermined voltage. As is describedbelow in detail, upon the voltage supply circuit 32 changing the voltageapplied to the photoelectric conversion units 13 among the plurality ofvoltages different from one another, the accumulation of the signalcharge carriers generated by the photoelectric conversion unit 13 at thecharge accumulation node 41 is started or terminated. In other words, inthis embodiment, upon the voltage supply circuit 32 changing the voltageapplied to the photoelectric conversion unit 13 from the third voltageto another voltage, the action of an electronic shutter is executed. Anexample of the action of the imaging apparatus 100 is described below.

The unit pixels 10 are each connected to a power source line 40, throughwhich a power source voltage VDD is supplied. As illustrated in FIG. 1,the power source line 40 is connected to the input terminal (typically,the drain) of the signal detection transistor 24. The power source line40 serves as a source-follower power source, which enables the signaldetection transistor 24 to amplify the signal charge carriers generatedby the photoelectric conversion unit 13 to a voltage appropriate to thesignal charge carriers and to output the amplified signal as a signalvoltage.

The output terminal (i.e., the source) of the signal detectiontransistor 24 is connected to the input terminal (i.e., the drain) ofthe address transistor 26. The output terminal (i.e., the source) of theaddress transistor 26 is connected to the corresponding one of aplurality of vertical signal lines 47, which are provided for therespective columns of the pixel array 200. The control terminal (i.e.,the gate) of the address transistor 26 is connected to the correspondingone of address control lines 46. Controlling the potentials of theaddress control lines 46 enables the data output from the signaldetection transistors 24 to be each selectively read through thecorresponding one of the vertical signal lines 47.

In the example illustrated in FIG. 1, the address control lines 46 areconnected to a vertical scanning circuit 36 (also referred to as “rowscanning circuit”). The vertical scanning circuit 36 selects a pluralityof the unit pixels 10 disposed in each row on a row-by-row basis byapplying a predetermined voltage to the corresponding one of the addresscontrol lines 46. This enables the readout of the signals from theselected unit pixels 10.

The vertical signal lines 47 are main signal lines through which pixelsignals output from the pixel array 200 are transmitted to theperipheral circuits. The vertical signal lines 47 are each connected tothe corresponding one of column-signal processing circuits 37 (alsoreferred to as “row-signal accumulation circuits”). The column-signalprocessing circuits 37 perform noise-reduction signal processing, suchas correlated double sampling, analog-digital conversion (ADconversion), and the like. As illustrated in FIG. 1, the column-signalprocessing circuits 37 are provided for the respective columns of theunit pixels 10 in the pixel array 200. The column-signal processingcircuits 37 are connected to a horizontal signal readout circuit 38(also referred to as “column-scanning circuit”), which sequentiallyreads a signal from each of the column-signal processing circuits 37 toa horizontal common signal line 49.

In the example structure illustrated in FIG. 1, the unit pixels 10 eachinclude a reset transistor 28. The reset transistor 28 is a field-effecttransistor or the like, similarly to the signal detection transistor 24and the address transistor 26. In the example described below, the resettransistor 28 is an N-channel MOSFET unless otherwise specified. Asillustrated in FIG. 1, the reset transistor 28 is connected to a resetvoltage line 44, through which a reset voltage Vr is supplied, and tothe charge accumulation node 41. The control terminal (i.e., the gate)of the reset transistor 28 is connected to the corresponding one ofreset control lines 48, and the potential of the charge accumulationnode 41 can be reset to the reset voltage Vr by controlling thepotential of the reset control line 48. In this example, the resetcontrol lines 48 are connected to the vertical scanning circuit 36.Thus, it is possible to reset a plurality of the unit pixels 10 whichare disposed in each row on a row-by-row basis by the vertical scanningcircuit 36 applying a predetermined voltage to the corresponding one ofthe reset control lines 48.

In this example, the reset voltage line 44, through which the resetvoltage Vr is supplied to the reset transistors 28, is connected to areset voltage source 34. The reset voltage source 34 may have anystructure that allows a predetermined reset voltage Vr to be suppliedthrough the reset voltage line 44 during the operation of the imagingapparatus 100 and is not limited to a specific power source circuit,similarly to the voltage supply circuit 32 described above. The voltagesupply circuit 32 and the reset voltage source 34 may be parts of asingle voltage supply circuit or independent voltage supply circuits.One or both of the voltage supply circuit 32 and the reset voltagesource 34 may be a part of the vertical scanning circuit 36.Alternatively, a sensitivity control voltage may be applied by thevoltage supply circuit 32 to the unit pixels 10 via the verticalscanning circuit 36, and/or the reset voltage Vr may be applied by thereset voltage source 34 to the unit pixels 10 via the vertical scanningcircuit 36.

The power source voltage VDD supplied to the signal detection circuits14 may be used also as a reset voltage Vr. In such a case, the resetvoltage source 34 may be used also as a voltage supply circuit (notillustrated in FIG. 1) that supplies a power source voltage to the unitpixels 10. Furthermore, it is possible to use the power source line 40also as the reset voltage line 44, which allows the arrangement of wiresin the pixel array 200 to be simplified. However, setting the resetvoltage Vr to be different from the power source voltage VDD supplied tothe signal detection circuits 14 increases the degree of flexibility inthe control of the imaging apparatus 100.

Device Structure of Unit Pixel

FIG. 2 schematically illustrates an exemplary device structure of theunit pixels 10. In the exemplary structure illustrated in FIG. 2, theabove-described signal detection transistor 24, the address transistor26, and the reset transistor 28 are disposed on a semiconductorsubstrate 20. The semiconductor substrate 20 is not limited to asubstrate the entirety of which is composed of a semiconductor and maybe an insulating substrate that includes a semiconductor layer disposedon a surface thereof on which a photosensitive region is to be formed.In the example described below, a p-type silicon (Si) substrate is usedas a semiconductor substrate 20.

The semiconductor substrate 20 includes impurity regions (in thisexample, N-type regions) 26 s, 24 s, 24 d, 28 d, and 28 s. Thesemiconductor substrate 20 also includes element separation regions 20 tin order to electrically separate the unit pixels 10 from one another.In this example, an element separation region 20 t is interposed alsobetween the impurity regions 24 d and 28 d. The element separationregions 20 t can be formed by, for example, injecting acceptor ions intothe semiconductor substrate 20 under predetermined injection conditions.

The impurity regions 26 s, 24 s, 24 d, 28 d, and 28 s are typicallydiffusion layers formed in the semiconductor substrate 20. Asschematically illustrated in FIG. 2, the signal detection transistor 24includes impurity regions 24 s and 24 d and a gate electrode 24 g(typically, a polysilicon electrode). The impurity regions 24 s and 24 dserve as, for example, the source region and the drain region,respectively, of the signal detection transistor 24. The channel regionof the signal detection transistor 24 is formed between the impurityregions 24 s and 24 d.

Similarly to the signal detection transistor 24, the address transistor26 includes impurity regions 26 s and 24 s and a gate electrode 26 g(typically, a polysilicon electrode), which is connected to thecorresponding one of the address control lines 46 (see FIG. 1). In thisexample, the signal detection transistor 24 and the address transistor26 are electrically connected to each other by sharing the impurityregion 24 s. The impurity region 26 s serves as, for example, the sourceregion of the address transistor 26. The impurity region 26 s isconnected to the corresponding one of vertical signal lines 47, which isnot illustrated in FIG. 2 (see FIG. 1).

The reset transistor 28 includes impurity regions 28 d and 28 s and agate electrode 28 g (typically, a polysilicon electrode) connected tothe corresponding one of reset control lines 48 (see FIG. 1). Theimpurity region 28 s serves as, for example, the source region of thereset transistor 28. The impurity region 28 s is connected to the resetvoltage line 44, which is not illustrated in FIG. 2 (see FIG. 1).

An interlayer insulating layer 50 (typically, a silicon dioxide layer)is disposed on the semiconductor substrate 20 so as to cover the signaldetection transistor 24, the address transistor 26, and the resettransistor 28. The interlayer insulating layer 50 may include a wiringlayer 56 formed therein as illustrated in FIG. 2. The wiring layer 56 istypically composed of a metal, such as copper, and may include wiressuch as the vertical signal lines 47 described above. The number ofinsulating sublayers constituting the interlayer insulating layer 50 andthe number of wiring sublayers constituting the wiring layer 56 formedin the interlayer insulating layer 50 may be set appropriately and notlimited to those in the example illustrated in FIG. 2.

The above-described photoelectric conversion unit 13 is disposed on theinterlayer insulating layer 50. In other words, in this embodiment, aplurality of unit pixels 10 constituting a pixel array 200 (see FIG. 1)are formed on the semiconductor substrate 20. The unit pixels 10, whichare arranged on the semiconductor substrate 20 in a two-dimensionalarray, form a photosensitive region. The distance (i.e., pixel pitch)between a pair of adjacent unit pixels 10 is, for example, about 2 μm.

The photoelectric conversion unit 13 includes a pixel electrode 11, acounter electrode 12, and a photoelectric conversion layer 15 interposedtherebetween. In this example, the counter electrode 12 and thephotoelectric conversion layer 15 are formed so as to extend across aplurality of the unit pixels 10.

On the other hand, each of the unit pixels 10 is provided with one pixelelectrode 11. Each of the pixel electrodes 11 is electrically separatedfrom other pixel electrodes 11 included in the adjacent unit pixels 10by being spatially separated from them.

The counter electrode 12 is typically a transparent electrode composedof a transparent conducting material. The counter electrode 12 isdisposed on a side of the photoelectric conversion layer 15 on whichlight enters. That is, light that have permeated through the counterelectrode 12 enters the photoelectric conversion layer 15. Thewavelength of light that can be detected by the imaging apparatus 100 isnot limited to be within the wavelength range (e.g., 380 nm or more and780 nm or less) of visible light. The term “transparent” used hereinrefers to passing at least part of light having a wavelength that fallswithin the detectable wavelength range. Hereinafter, all electromagneticwaves including infrared radiation and ultraviolet radiation arecollectively referred to as “light” for the sake of convenience. Thecounter electrode 12 may be composed of, for example, a transparentconducting oxide (TCO), such as indium-doped tin oxide (ITO), indiumzinc oxide (IZO), aluminium-doped zinc oxide (AZO), fluorine-doped tinoxide (FTO), SnO₂, TiO₂, or ZnO₂.

The photoelectric conversion layer 15 generates electron-hole pairs uponreceiving the incident light. In this embodiment, the photoelectricconversion layer 15 is composed of an organic material. Specificexamples of the material for the photoelectric conversion layer 15 aredescribed below.

As described above with reference to FIG. 1, the counter electrode 12 isconnected to the corresponding one of the sensitivity control lines 42,which are connected to the voltage supply circuit 32. In thisembodiment, the counter electrode 12 is formed so as to extend across aplurality of the unit pixels 10. This enables the voltage supply circuit32 to apply a desired sensitivity control voltage to a plurality of theunit pixels 10 at a time through the sensitivity control lines 42. Eachof the unit pixels 10 may be provided with one counter electrode 12 aslong as a desired sensitivity control voltage can be applied from thevoltage supply circuit 32. Similarly, each of the unit pixels 10 may beprovided with one photoelectric conversion layer 15.

As described below in detail, the voltage supply circuit 32 appliesdifferent voltages to the counter electrode 12 during the exposureperiod and the non-exposure period. The term “exposure period” usedherein refers to the period in which positive or negative chargecarriers generated by the photoelectric conversion are accumulated atthe charge accumulation region and may be referred to also as “chargeaccumulation period”. The term “non-exposure period” used herein refersto the period in which the imaging apparatus is operated and which isother than the exposure period. The “non-exposure period” is not limitedto the period during which the photoelectric conversion unit 13 is notirradiated with light and may include the period during which thephotoelectric conversion unit 13 is irradiated with light.

Controlling the potential of the counter electrode 12 with respect tothe pixel electrode 11 enables holes or electrons of the electron-holepairs generated in the photoelectric conversion layer 15 byphotoelectric conversion to be collected by the pixel electrode 11. Forexample, in the case where holes are used as signal charge carriers,setting the potential of the counter electrode 12 to be higher than thatof the pixel electrode 11 enables the holes to be selectively collectedby the pixel electrode 11. In the example described below, holes areused as signal charge carriers. Needless to say that electrons mayalternatively be used as signal charge carriers.

The pixel electrode 11, which faces the counter electrode 12, collectspositive or negative charge carriers generated in the photoelectricconversion layer 15 by photoelectric conversion upon an appropriate biasvoltage being applied between the counter electrode 12 and the pixelelectrode 11. The pixel electrode 11 is composed of a metal, such asaluminium or copper, a nitride of the metal, or a polysilicon or thelike which is made conductive by being doped with an impurity.

The pixel electrode 11 may have a light-blocking property. When thepixel electrode 11 is, for example, a TaN electrode having a thicknessof 100 nm, the pixel electrode 11 may have a sufficient light-blockingproperty. Using an electrode having a light-blocking property as a pixelelectrode 11 may reduce the intrusion of light that permeates throughthe photoelectric conversion layer 15 into the channel regions or theimpurity regions of the transistors (in this example, at least one ofthe signal detection transistor 24, the address transistor 26, and thereset transistor 28) formed on the semiconductor substrate 20. Alight-blocking film may optionally be formed in the interlayerinsulating layer 50 by using the wiring layer 56 described above.Reducing the intrusion of the light into the channel regions of thetransistors formed on the semiconductor substrate 20 may limit a shiftof the properties of the transistors (e.g., the fluctuations inthreshold voltage). Reducing the intrusion of the light into theimpurity regions formed on the semiconductor substrate 20 may limit themixing of noises generated as a result of unintended photoelectricconversion occurring in the impurity regions. Thus, reducing theintrusion of the light into the semiconductor substrate 20 increases thereliability of the imaging apparatus 100.

As schematically illustrated in FIG. 2, the pixel electrode 11 isconnected to the gate electrode 24 g of the signal detection transistor24 with a plug 52, a wire 53, and a contact plug 54. In other words, thegate of the signal detection transistor 24 is electrically connectedwith the pixel electrode 11. The plug 52 and the wire 53 are composedof, for example, a metal, such as copper. The plug 52, the wire 53, andthe contact plug 54 constitute at least a part of the chargeaccumulation node 41 (see FIG. 1), which is located between the signaldetection transistor 24 and the photoelectric conversion unit 13. Thewire 53 may constitute a part of the wiring layer 56. The pixelelectrode 11 is also connected to the impurity region 28 d with the plug52, the wire 53, and a contact plug 55. In the exemplary structureillustrated in FIG. 2, the gate electrode 24 g of the signal detectiontransistor 24, the plug 52, the wire 53, the contact plugs 54 and 55,and the impurity region 28 d, which serves as a source or drain regionof the reset transistor 28, serve as a charge accumulation region atwhich the signal charge carriers collected by the pixel electrode 11 areaccumulated.

Upon the signal charge carriers being collected by the pixel electrode11, a voltage responsive to the amount of signal charge carriersaccumulated at the charge accumulation region is applied to the gate ofthe signal detection transistor 24. The signal detection transistor 24amplifies the voltage. The voltage amplified by the signal detectiontransistor 24 is selectively read as a signal voltage via the addresstransistor 26.

Example Structure of Photoelectric Conversion Layer

As described above, when the photoelectric conversion layer 15 isirradiated with light and a bias voltage is applied between the pixelelectrode 11 and the counter electrode 12, positive or negative chargecarriers generated by photoelectric conversion are collected by thepixel electrode 11 and the collected charge carriers are accumulated atthe charge accumulation region. In this embodiment, the photoelectricconversion unit 13 includes a photosensitive layer 15A including twophotoelectric conversion films that have different absorption spectraand stacked on top of each other under specific conditions, which isdescribed below. This enables the wavelength sensitivity characteristicof the photoelectric conversion layer 15, that is, the imagingwavelength range, to be changed by controlling the difference inpotential between the pixel electrode 11 and the counter electrode 12.Furthermore, limiting the difference in potential between the pixelelectrode 11 and the counter electrode 12 to be equal to or smaller thana predetermined potential prevents the migration of the signal chargecarriers accumulated at the charge accumulation region into the counterelectrode 12 through the photoelectric conversion layer 15. Limiting thedifference in potential between the pixel electrode 11 and the counterelectrode 12 to be equal to or smaller than a predetermined potentialalso reduces the likelihood of the signal charge carriers being furtheraccumulated at the charge accumulation region. The above is one of thefindings made by the inventors. That is, it is possible to achieve aglobal shutter and electrical switchover of the imaging wavelength bycontrolling the bias voltage applied to the photoelectric conversionlayer 15 without optional elements such as transfer transistors beingdisposed on the respective pixels. A typical example of the action ofthe imaging apparatus 100 is described below.

An example structure of the photoelectric conversion layer 15 and thewavelength sensitivity characteristic of the photoelectric conversionlayer 15 are described below.

FIG. 3 is a schematic cross-sectional view of a photoelectric conversionlayer 15, illustrating an example structure of the photoelectricconversion layer 15. In the structure illustrated in FIG. 3, thephotoelectric conversion layer 15 includes a hole-blocking layer 15 h,the photosensitive layer 15A, and an electron-blocking layer 15 e. Thehole-blocking layer 15 h is interposed between the photosensitive layer15A and the counter electrode 12. The electron-blocking layer 15 e isinterposed between the photosensitive layer 15A and the pixel electrode11. The photoelectric conversion layer 15 typically includes asemiconductor material. In this embodiment, an organic semiconductormaterial is used as a semiconductor material. The organic semiconductormaterial may be any organic semiconductor material that has absorptionpeaks at imaging wavelengths required for imaging. Common photoelectricconversion films are produced from a mixture of an electron-donatingorganic semiconductor material and an electron-accepting organicsemiconductor material. This enables the strength of electric fieldrequired for separating excitons generated in the photoelectricconversion layer 15 into electrons and holes to be reduced to a leveladequate for use as a semiconductor element. In other words, the quantumefficiency at the same voltage may be increased, and the photoelectricconversion characteristic may be improved.

In this embodiment, a first photoelectric conversion film 150 a disposedon the counter-electrode-12-side has a wavelength sensitivitycharacteristic such that the first photoelectric conversion film 150 ais sensitive only in the visible region, and the second photoelectricconversion film 150 b disposed on the pixel-electrode-11-side has awavelength sensitivity characteristic such that the second photoelectricconversion film 150 b has a sensitivity in the visible region and ahigher sensitivity in the infrared region than in the visible region.Hereinafter, having a sensitivity lower than one tenth of the peakwavelength is considered to be having no sensitivity.

Since the photoelectric conversion layer 15 is composed of organicsemiconductor materials, it is possible to control the amount of signalcharge carriers that reach the pixel electrode 11 by changing thevoltage applied between the counter electrode 12 and the pixel electrode11. This is because an organic semiconductor material has a considerablylow carrier transporting capability and, therefore, the charge carriersgenerated in the photoelectric conversion layer 15 may be lost as aresult of, for example, recombination while migrating through thephotoelectric conversion layer 15 when the strength of the electricfield that attracts the charge carriers toward the pixel electrode 11 isnot large.

In this embodiment, the first photoelectric conversion film 150 a andthe second photoelectric conversion film 150 b that have differentabsorption spectra are interposed between the counter electrode 12 andthe pixel electrode 11. This enables the amount of signal chargecarriers transported to the pixel electrode 11 to be controlled inaccordance with the voltage applied between the counter electrode 12 andthe pixel electrode 11 only when the first photoelectric conversion film150 a and the second photoelectric conversion film 150 b satisfypredetermined conditions. This is also one of the findings made by theinventors.

An example of the predetermined conditions is to make the resistances ofthe first photoelectric conversion film 150 a and the secondphotoelectric conversion film 150 b different from each other by acertain degree or more. The voltage applied between the counterelectrode 12 and the pixel electrode 11 is distributed at the ratiobetween the resistances of the first photoelectric conversion film 150 aand the second photoelectric conversion film 150 b. Making theresistances of the first photoelectric conversion film 150 a and thesecond photoelectric conversion film 150 b different from each other by,for example, 44 times or more enables most of the voltage appliedbetween the electrodes to be distributed to the photoelectric conversionfilm having a higher resistance and the voltage distributed to the otherphotoelectric conversion film to be reduced to the photoelectricconversion threshold or less. Therefore, when the voltage appliedbetween the counter electrode 12 and the pixel electrode 11 is low, anelectric field equal to or higher than the photoelectric conversionthreshold is applied only to the photoelectric conversion film having ahigher resistance, and only the signal charge carriers generated in thephotoelectric conversion film having a higher resistance are transportedto the pixel electrode 11. When the voltage applied between the counterelectrode 12 and the pixel electrode 11 is increased, an electric fieldequal to or higher than the photoelectric conversion threshold isapplied also to the photoelectric conversion film having a lowerresistance, and the photoelectric conversion film having a lowerresistance becomes capable of transporting the signal charge carriers tothe pixel electrode 11. The resistance of each of the photoelectricconversion films can be adjusted by changing the energy levels of anelectron-donating material and an electron-accepting material thatconstitute the photoelectric conversion film, the mixing ratio betweenthe electron-donating material and the electron-accepting material, thethickness of the photoelectric conversion film, and the like.

When the voltage applied to the photoelectric conversion layer 15 (i.e.,the voltage applied between the counter electrode 12 and the pixelelectrode 11) is lower than both of the photoelectric conversionthresholds of the first photoelectric conversion film 150 a and thesecond photoelectric conversion film 150 b stacked on top of each other,both photoelectric conversion layers become incapable of transportingthe signal charge carriers to the pixel electrode. As a result, thewavelength sensitivities of the first photoelectric conversion film 150a and the second photoelectric conversion film 150 b becomesubstantially zero, and the first photoelectric conversion film 150 aand the second photoelectric conversion film 150 b become insensitive.Thus, by selecting the voltage applied between the counter electrode 12and the pixel electrode 11, it is possible to perform a switchover amongthe state in which the photoelectric conversion films do not have aphotoelectric conversion sensitivity over the entire wavelength range,the state in which only one of the first photoelectric conversion film150 a and the second photoelectric conversion film 150 b constitutingthe photosensitive layer 15A has a photoelectric conversion sensitivity,and the state in which both of the first photoelectric conversion film150 a and the second photoelectric conversion film 150 b constitutingthe photosensitive layer 15A have a photoelectric conversionsensitivity. As described above, it is possible to change the wavelengthsensitivity characteristic of the photoelectric conversion layer 15,that is, the imaging wavelength range, and to execute the action of anelectronic shutter by changing the difference in potential between thepixel electrode 11 and the counter electrode 12.

The first photoelectric conversion film 150 a and the secondphotoelectric conversion film 150 b may be, for example, a film formedby the codeposition of2-{[7-(5-N,N-ditolylaminothiophen-2-yl)-2,1,3-benzothiadiazol-4-yl]methylene}malononitrile(DTDCTB) and C₇₀ fullerene or a film formed by the codeposition of tinnaphthalocyanine and C₇₀ fullerene. Specifically, the firstphotoelectric conversion film 150 a and the second photoelectricconversion film 150 b may be composed of the following materials. Thefirst photoelectric conversion film 150 a and the second photoelectricconversion film 150 b typically each include an electron-donating (i.e.,donor-type or p-type) molecule and an electron-accepting (i.e.,acceptor-type or n-type) molecule.

A typical example of the electron-donating molecule is a p-type organicsemiconductor which is likely to donate electrons, such as ahole-transporting organic compound. Examples of the p-type organicsemiconductor include triarylamines, such as DTDCTB; benzidines;pyrazolines; styrylamines; hydrazones; triphenylmethanes; carbazoles;polysilanes; thiophenes, such as α-sexithiophene (α-6T) andpoly-3-hexylthiophene (P3HT); phthalocyanines; cyanines; merocyanines;oxonols; polyamines; indoles; pyrroles; pyrazoles; polyarylenes;condensed aromatic carbocyclic compounds (e.g., a naphthalenederivative, an anthracene derivative, a phenanthrene derivative, atetracene derivative, such as rubrene, a pyrene derivative, a perylenederivative, and a fluoranthene derivative); and metal complexesincluding a ligand that is a nitrogen-containing heterocyclic compound.Examples of the phthalocyanines include copper phthalocyanine (CuPc),subphthalocyanine (SubPc), aluminum chloride phthalocyanine (ClAlPc),Si(OSiR₃)₂Nc (where R represents an alkyl group having 1 to 18 carbonatoms and Nc represents naphthalocyanine), tin naphthalocyanine (SnNc),and lead phthalocyanine (PbPc). Examples of the donor-type organicsemiconductor are not limited to the above compounds. Any organiccompound having a lower ionization potential than an organic compoundused as an n-type (acceptor-type) organic compound may be used as adonor-type organic semiconductor. The ionization potential is thedifference in energy level between the vacuum level and the highestoccupied molecular orbital (HOMO).

A typical example of the electron-accepting molecule is an n-typeorganic semiconductor that is likely to accept electrons, such as anelectron-transporting organic compound. Examples of the n-type organicsemiconductor include fullerene (e.g., C₆₀ or C₇₀), fullerenederivatives (e.g., phenyl-C₆₁-butyric acid methyl ester (PCBM)),condensed aromatic carbocyclic compounds (e.g., a naphthalenederivative, an anthracene derivative, a phenanthrene derivative, atetracene derivative, a pyrene derivative, a perylene derivative, and afluoranthene derivative), five- or seven-membered heterocyclic compoundscontaining a nitrogen atom, an oxygen atom, or a sulfur atom (e.g.,pyridine, pyradine, pyrimidine, pyridadine, triazine, quinoline,quinoxaline, quinazoline, phthalazine, cinnoline, isoquinoline,pteridine, acridine, phenazine, phenanthroline, tetrazole, pyrazole,imidazole, thiazole, oxazole, indazole, benzimidazole, benzotriazole,benzoxazole, benzothiazole, carbazole, purine, triazolopyridadine,triazolopyrimidine, tetrazaindene, oxadiazole, imidazopyridine,pyrrolidine, pyrrolopyridine, thiadiazolopyridine, dibenzazepine, andtribenzazepine), subphthalocyanines (SubPc), polyarylenes, fluorenes,cyclopentadienes, silyl compounds, perylenetetracarboxylic diimides(PTCDl), and metal complexes including a nitrogen-containingheterocyclic compound as a ligand. Examples of the acceptor-type organicsemiconductor are not limited to the above compounds. Any organiccompound having a larger electron affinity than an organic compound usedas a p-type (donor-type) organic compound may be used as anacceptor-type organic semiconductor. The electron affinity is thedifference in energy level between the vacuum level and the lowestunoccupied molecular orbital (LUMO).

The first photoelectric conversion film 150 a may be composed of, forexample, an organic semiconductor material having a photoelectricconversion sensitivity in the visible region. The second photoelectricconversion film 150 b may be composed of, for example, an organicsemiconductor material having a sensitivity in the infrared region.

The above-described advantageous effects may also be achieved bycreating an energy barrier at the interface between the firstphotoelectric conversion film 150 a and the second photoelectricconversion film 150 b of the photosensitive layer 15A. For example, inthe case where holes are to be accumulated as signal charge carriers,the ionization potentials of the first photoelectric conversion film 150a and the second photoelectric conversion film 150 b are selected suchthat an energy barrier of, for example, 0.2 eV or more to holes iscreated at the interface between the first photoelectric conversion film150 a and the second photoelectric conversion film 150 b of thephotosensitive layer 15A.

In the case where an energy barrier is not created at the interfacebetween the first photoelectric conversion film 150 a and the secondphotoelectric conversion film 150 b, the voltage dependence of thewavelength sensitivity characteristic varies with the resistances of thefirst photoelectric conversion film 150 a and the second photoelectricconversion film 150 b, and the order in which the photoelectricconversion films are stacked on top of each other does not affect theaction of switchover of the wavelength sensitivity characteristic.However, there are preferable stacking conditions in terms of theefficiency of photoelectric conversion. As described above, thephotoelectric conversion films are commonly composed of a mixture of anelectron-donating organic semiconductor material and anelectron-accepting organic semiconductor material. Theelectron-accepting material may be C₆₀ fullerene, C₇₀ fullerene, or ananalog to these fullerenes. This is because the LUMO of fullerenespreads spatially in the form of a spherical shell, which enableselectrons to migrate from an electron-donating organic semiconductormaterial that is brought into contact with the electron-acceptingmaterial into fullerene molecules with high efficiency. Accordingly, inorder to produce a high-efficiency photoelectric conversion element, thephotoelectric conversion films may be formed using a mixture of anelectron-donating organic semiconductor material having an absorption atdesired wavelengths and fullerene or an analog to fullerene. It is knownthat fullerene and analogs to fullerene commonly have a strongabsorption in the visible region and, in particular, at wavelengthscorresponding to blue. Therefore, an infrared photoelectric conversionfilm composed of an electron-donating organic semiconductor materialhaving an absorption at the infrared region has an absorption in notonly the infrared region but also the visible region. In such a case, ifthe photoelectric conversion films are stacked on top of each other suchthat the first photoelectric conversion film 150 a is the infraredphotoelectric conversion film and the second photoelectric conversionfilm 150 b is a visible photoelectric conversion film sensible in thevisible region, the first photoelectric conversion film disposed on thelight-incident side absorbs part of the visible light and reduces theamount of visible light incident on the second photoelectric conversionfilm, which is a visible photoelectric conversion film. Therefore, insuch a case, using the visible photoelectric conversion film and theinfrared photoelectric conversion film as the first photoelectricconversion film 150 a and the second photoelectric conversion film 150b, respectively, may limit the reduction in the amount of visible lightincident on the second photoelectric conversion film. On the other hand,a visible photoelectric conversion film composed of an electron-donatingorganic semiconductor capable of absorbing visible light and fullereneor an analog to fullerene does not have an absorption in the infraredregion. Therefore, even when the above visible photoelectric conversionfilm is disposed on the light-incident side, it allows infrared light totransmit therethrough and enables a sufficient amount of infrared lightto reach the infrared photoelectric conversion film. As described above,in order to perform imaging in a suitable manner, the order in which theplurality of photoelectric conversion films are stacked on top of oneanother may be determined such that a sufficient amount of light ofdesired wavelengths reaches a desired photoelectric conversion film.

Example Action of Imaging Apparatus 100

FIG. 4 includes timing charts for explaining an example of the action ofthe imaging apparatus according to an embodiment of the presentdisclosure. FIGS. 4(a) and 4(b) each illustrate the timing of the riseor fall of a synchronizing signal. FIG. 4(c) illustrates a change in thebias voltage applied to the photoelectric conversion layer 15 with time.FIG. 4(d) illustrates the timing of the reset and exposure of each rowof the pixel array 200 (see FIG. 1). Specifically, FIG. 4(a) illustratesthe timing of the rise or fall of a vertical synchronizing signal Vss.FIG. 4(b) illustrates the timing of the rise or fall of a horizontalsynchronizing signal Hss. FIG. 4(c) illustrates a change, with time, inthe voltage Vb applied from the voltage supply circuit 32 to the counterelectrodes 12 via the sensitivity control lines 42. FIG. 4(d)illustrates the timing of the reset and exposure of each row of thepixel array 200. The change, with time, in the voltage Vb applied fromthe voltage supply circuit 32 to the counter electrodes 12 via thesensitivity control lines 42 corresponds to a change, with time, in thepotential of the counter electrode 12 with respect to the potential ofthe pixel electrode 11.

While FIG. 4 illustrates an example case where the pixel array 200includes unit pixels 10 arranged in three rows for the sake ofsimplicity, the unit pixels included in the pixel array 200 may bearranged in four or more rows.

An example of the action of the imaging apparatus 100 is described belowwith reference to FIGS. 1, 2, and 4. For the sake of simplicity, anexample of the action of an imaging apparatus 100 that includes a pixelarray 200 including pixels arranged in three rows in total, that is, therow <0>, the row <1>, and the row <2>, is described below.

For acquiring an image, first, the charge accumulation region of each ofthe unit pixels 10 included in the pixel array 200 is reset and, whenthe pixel array 200 has been exposed to light prior to resetting, apixel signal is read from each of the reset unit pixels 10. For example,as illustrated in FIG. 4, the resetting of a plurality of pixels in therow <0> is started in response to the vertical synchronizing signal Vss(Time t0). In FIG. 4, the shaded rectangular portions schematicallyrepresent a period in which a signal is read. The readout periodincludes a resetting period in which the potential of the chargeaccumulation region of each unit pixel 10 is reset.

For resetting the pixels in the row <0>, the address transistors 26whose gates are connected to the specific one of the address controllines 46 which is associated with the row <0> are turned on bycontrolling the potential of the address control line 46. Furthermore,the reset transistors 28 whose gates are connected to the specific oneof the reset control lines 48 which is associated with the row <0> areturned on by controlling the potential of the reset control line 48.Thus, the charge accumulation node 41 and the reset voltage line 44 areconnected to each other, and a reset voltage Vr is supplied to thecharge accumulation node 41, which is the charge accumulation region.Specifically, the potential of the gate electrode 24 g of each signaldetection transistor 24 and the potential of the pixel electrode 11 ofeach photoelectric conversion unit 13 are reset to be the reset voltageVr. Subsequently, a pixel signal is read from each of the reset unitpixels 10 in the row <0> via the corresponding one of the verticalsignal lines 47. These pixel signals are responsive to the reset voltageVr. Subsequent to the readout of the pixel signals, the resettransistors 28 and the address transistors 26 are turned off.

In this example, the pixels in each of the row <0>, the row <1>, and therow <2> are reset sequentially on a row-by-row basis in response to thehorizontal synchronizing signal Hss as schematically illustrated in FIG.4. Hereinafter, the intervals between the pulses of the horizontalsynchronizing signal Hss, that is, the period from when a row isselected to when the next row is selected, is referred to as “1Hperiod”. In this example, the period between Time t0 and Time t1corresponds to the 1H period.

As illustrated in FIG. 4, a third voltage V3 at which the sensitivity ofthe photoelectric conversion layer 15 in a predetermined wavelengthrange becomes substantially zero is applied by the voltage supplycircuit 32 between the pixel electrode 11 and the counter electrode 12during at least a resetting period in which all of the chargeaccumulation regions included in the respective unit pixels 10 of thepixel array 200 are reset and the period (Time t0 to Time t3) from thestart of the acquisition of the image to the end of the resetting of allthe rows of the pixel array 200 and the readout of pixel signals. Thethird voltage V3 is, for example, 0 volt (V). In other words, applying abias voltage, that is, the third voltage V3, to the photoelectricconversion layer 15 included in the photoelectric conversion unit 13creates the period from the timing (Time t0) of the start of theacquisition of the image signal from the pixel array 200 to the start(Time t3) of the exposure period, that is, the non-exposure period.

While the bias voltage, that is, the third voltage V3, is applied to thephotoelectric conversion layer 15, the migration of signal chargecarriers from the photoelectric conversion layer 15 to the chargeaccumulation region is negligible. This is presumably because, while thebias voltage, that is, the third voltage V3, is applied to thephotoelectric conversion layer 15, positive and negative charge carriersgenerated as a result of the photoelectric conversion layer 15 beingirradiated with light quickly recombine with each other and disappearbefore collected by the pixel electrode 11. Accordingly, while the biasvoltage, that is, the third voltage V3, is applied to the photoelectricconversion layer 15, the accumulation of signal charge carriers at thecharge accumulation region is negligible even when the photoelectricconversion layer 15 is irradiated with light. This reduces theoccurrence of unintended sensitivity in the non-exposure period(hereinafter, this sensitivity is referred to as “parasiticsensitivity”).

In a row (e.g., the row <0>) of FIG. 4(d), the periods represented bythe heavily shaded rectangular portion and the lightly shadedrectangular portion are non-exposure periods. The third voltage V3 isnot limited to be 0 V.

Subsequent to the resetting of all the rows of the pixel array 200 andthe readout of pixel signals, exposure is started in response to thehorizontal synchronizing signal Hss (Time t3). In FIG. 4(d), the blankrectangular portions schematically represent the exposure period in eachrow. In FIG. 4, the non-exposure periods denoted by the shaded rectangleportions have the same length as the exposure periods denoted by theblank rectangular portions for the sake of simplicity; the non-exposureperiods and the exposure periods do not necessarily have the samelength. The length of the exposure periods may be adjusted adequately inaccordance with the darkness of the subject, the speed of motion of thesubject, and the like. The exposure period starts upon the voltagesupply circuit 32 changing the voltage applied between the pixelelectrode 11 and the counter electrode 12 to a first voltage V1 (or asecond voltage V2) which is different from the third voltage V3. Thefirst voltage V1 is, for example, the voltage at which the photoelectricconversion thresholds of all the sublayers of the photosensitive layer15A are reached and all the sublayers of the photosensitive layer 15Acome to have a sensitivity. Upon the first voltage V1 being applied tothe photoelectric conversion layer 15, the signal charge carriers (inthis example, holes) generated in the photoelectric conversion layer 15are collected by the pixel electrode 11 and accumulated at the chargeaccumulation region. FIG. 5 illustrates an example of the voltagedependence of the wavelength sensitivity characteristic of thephotoelectric conversion layer 15 included in the imaging apparatus 100according to the embodiment. In this embodiment, signal charge carriersgenerated by the photoelectric conversion of light in the visible andinfrared regions are accumulated at the charge accumulation region inaccordance with the wavelength sensitivity characteristic illustrated inFIG. 5. In FIG. 4(d), the periods denoted by the blank rectangularportions labeled as “RGB+IR” are the exposure periods during whichsignal charge carriers generated by the photoelectric conversion oflight in the visible and infrared regions are accumulated at the chargeaccumulation region.

The exposure period is terminated upon the voltage supply circuit 32changing the voltage applied between the pixel electrode 11 and thecounter electrodes 12 to the third voltage V3 (Time t6). Subsequently,signal charge is read from the pixels in each row of the pixel array 200in response to the horizontal synchronizing signal Hss. In this example,signal charge is read from the pixels in each of the row <0>, the row<1>, and the row <2> sequentially on a row-by-row basis from Time t6.Hereinafter, the period from when pixels in a row are selected to whenthe pixels in the row are again selected may be referred to as “1Vperiod”. In this example, the period from Time t0 to Time t6 correspondsto the 1V period. The 1V period corresponds to one frame. In theexposure period of an 1V period next to the 1V period in which the firstvoltage V1 is applied to the photoelectric conversion layer 15, thevoltage applied to the photoelectric conversion layer 15 is changed to asecond voltage V2 different from the first voltage V1 or the thirdvoltage V3. The second voltage V2 is, for example, a voltage at whichone of the photoelectric conversion thresholds of the sublayers of thephotosensitive layer 15A is reached. The second voltage V2 is, forexample, a voltage intermediate between the first voltage V1 and thethird voltage V3. In this embodiment, while the voltage applied to thephotoelectric conversion layer 15 is the second voltage V2, signalcharge carriers generated by the photoelectric conversion of only thelight in the visible region are accumulated at the charge accumulationregion as illustrated in FIG. 5. In FIG. 4(d), the periods denoted bythe blank rectangular portions labeled as “RGB” are the exposure periodsduring which signal charge carriers generated by the photoelectricconversion of only the light in the visible region are accumulated atthe charge accumulation region.

The exposure period is terminated when the voltage applied to thephotoelectric conversion layer 15 is changed to the third voltage V3(Time t12). Subsequently, signal charge is read from the pixels in eachrow of the pixel array 200 in response to the horizontal synchronizingsignal Hss.

In the above-described embodiment of the present disclosure, the voltageapplied between the pixel electrode 11 and the counter electrode 12 isset to the first voltage V1 or the second voltage V2 during the exposureperiod and to the third voltage V3 while the voltage applied to thephotoelectric conversion layer 15 is changed from the first voltage V1to the second voltage V2 or from the second voltage V2 to the firstvoltage V1, that is, during the non-exposure period that includes atleast the period in which signal charge is read from all the chargeaccumulation regions. This enables two images to be acquired from thephotoelectric conversion layer 15 with different wavelength sensitivitycharacteristics in a “global shutter” mode.

In addition, repeating an 1V period during which the first voltage V1 isapplied between the pixel electrode 11 and the counter electrode 12 andan 1V period during which the second voltage V2 is applied between thepixel electrode 11 and the counter electrode 12 in an alternating mannerenables acquisition of two moving images from the photoelectricconversion layer 15 with different wavelength sensitivitycharacteristics to be achieved substantially simultaneously with oneimaging apparatus 100. Since the two moving images are acquiredelectrically in a global shutter mode, the distortion of a fast-movingsubject can be reduced in any of the moving images.

The period during which the first voltage V1 is applied between thepixel electrode 11 and the counter electrode 12 and the period duringwhich the second voltage V2 is applied between the pixel electrode 11and the counter electrode 12 are not necessarily repeated alternately,and the switchover between the two voltages may be performed at anyfrequency. The frequency at which the switchover between the twovoltages is performed may be determined in accordance with the number offrames. For example, the switchover between the two voltages may beperformed once every several tens of frames. The switchover between thetwo voltages is not necessarily performed periodically and may beperformed at a timing based on a trigger. In another case, in order todetect infrared light at desired timings, imaging information based oninfrared light may be acquired, for example, only once a second in animaging mode in which visible light and infrared light are used, while acolor image is acquired, in normal times, in an imaging mode in whichvisible light is used.

In this embodiment, upon the first voltage V1 being applied to thephotoelectric conversion layer 15, the photoelectric conversion layer 15exhibits a wavelength sensitivity characteristic such that thephotoelectric conversion layer 15 has a sensitivity in the visibleregion and a sensitivity equal to or higher than a predetermined firstthreshold in the infrared region. Upon the second voltage V2 lower thanthe first voltage V1 being applied to the photoelectric conversion layer15, the photoelectric conversion layer 15 exhibits a wavelengthsensitivity characteristic such that the photoelectric conversion layer15 has a sensitivity equal to or higher than a predetermined secondthreshold only in the visible region. This enables the switchoverbetween an exposure period in which visible and infrared wavelengths areused and an exposure period in which only visible wavelengths are used.As is clear from FIG. 4, in this example, the initiation (Time t3 andTime t9) and the termination (Time t6 and Time t12) of the exposureperiod are each performed at the same timing over all the pixelsincluded in the pixel array 200. In other words, the action describedabove is a “global shutter” mode.

On the other hand, in the case where the voltage applied between thepixel electrode 11 and the counter electrode 12 is not set to the thirdvoltage V3 when it is changed between the first voltage V1 and thesecond voltage V2, it is not possible to electrically create thenon-exposure state. Consequently, the acquisition of images is done in a“rolling shutter” mode. Furthermore, the two types of signal chargecarriers which are generated in the photoelectric conversion layer 15with different wavelength sensitivity characteristics, that is, thesignal charge carriers resulting from only visible light and the signalcharge carriers resulting from light of visible and infraredwavelengths, are accumulated at the charge accumulation region in amixed state. This makes it difficult to acquire a correct image.

In this embodiment, subsequent to the termination of the exposureperiod, the address transistors 26 in the row <0> are turned on in orderto read signal charge from the pixels in the row <0>. This allows thepixel signals responsive to the amounts of charge carriers accumulatedat the respective charge accumulation regions during the exposure periodto be output through the vertical signal lines 47. Subsequent to thereadout of the pixel signals, the pixels may be reset by turning on thereset transistors 28 in order to read the reset voltage of the pixels asa reference signal of the pixels. Subsequent to the readout of the pixelsignals, the address transistors 26 (and the reset transistors 28) areturned off. Subsequent to the readout of the signal charge from thepixels in each row of the pixel array 200, the difference between thepixel signals read at Time t0 and the reference signals read at Time t3is determined in order to remove fixed noises contained in the signals.

In the above period, the third voltage V3 is applied between the pixelelectrode 11 and the counter electrode 12, that is, a bias voltage isapplied to the photoelectric conversion layer 15 such that thesensitivity of the photoelectric conversion layer 15 becomessubstantially zero. Therefore, in the above period, the accumulation ofsignal charge carriers at the charge accumulation region is negligibleeven when the photoelectric conversion layer 15 is irradiated withlight. This reduces the likelihood of noise being caused as a result ofunintended charge carriers mixing in the photoelectric conversion layer15.

Alternatively, in order to reduce the likelihood of the signal chargecarriers being further accumulated at the charge accumulation regionduring the non-exposure period, the exposure period may be terminated byapplying a voltage having the same magnitude as the third voltage V3 anda polarity opposite to the third voltage V3 between the pixel electrode11 and the counter electrode 12. However, simply reversing the polarityof the voltage applied between the pixel electrode 11 and the counterelectrode 12 may cause the signal charge carriers accumulated at thecharge accumulation region to migrate into the counter electrode 12through the photoelectric conversion layer 15. The migration of thesignal charge carriers is observed as black dots in the acquired image.That is, the migration of the signal charge carriers from the chargeaccumulation region to the counter electrode 12 through thephotoelectric conversion layer 15 during the non-exposure period mayresult in a negative parasitic sensitivity.

In this embodiment, the exposure period is terminated by changing thevoltage applied to the photoelectric conversion layer 15 to the thirdvoltage V3, that is, a bias voltage at which the sensitivity of thephotoelectric conversion layer 15 becomes substantially zero is appliedto the photoelectric conversion layer 15 subsequent to the accumulationof the signal charge carriers at the charge accumulation region. Whilethe third voltage V3 is applied to the photoelectric conversion layer 15as a bias voltage, the likelihood of the signal charge carriersaccumulated at the charge accumulation region migrating to the counterelectrode 12 through the photoelectric conversion layer 15 can bereduced. In other words, applying the third voltage V3 to thephotoelectric conversion layer 15 enables the signal charge carriersaccumulated during the exposure period to be stored at the chargeaccumulation region. That is, applying the third voltage V3 to thephotoelectric conversion layer 15 may reduce the likelihood of thenegative parasitic sensitivity being caused as a result of the signalcharge carriers being lost from the charge accumulation region.

As described above, in this embodiment, the initiation and terminationof the exposure period and the imaging wavelength during the exposureperiod are controlled by changing the voltage Vb applied between thepixel electrode 11 and the counter electrode 12. That is, according tothis embodiment, it is possible to perform imaging by simultaneouslychanging the imaging wavelength of each of the unit pixels 10.

Organic photoelectric conversion elements that include a photoelectricconversion layer including an organic semiconductor material have a moresimple structure and are capable of being produced by a more simpleprocess than inorganic photoelectric conversion elements that includephotodiodes which are known in the related art. In addition, it ispossible to readily design the wavelength range in which an organicsemiconductor material contributes to photoelectric conversion. Thismakes it possible to realize a photoelectric conversion element having adesired wavelength sensitivity characteristic.

In the case where an image sensor that includes a plurality of organicphotoelectric conversion elements is used, the sensitivity of the imagesensor can be changed for each exposure period by changing the voltagesapplied to the photoelectric conversion elements, but the imagingwavelength of the image sensor cannot be changed for each exposureperiod. One of the methods for changing the imaging wavelength is todetachably attach a filter that transmits only light having desiredwavelengths onto the front surface of the image sensor. However, in sucha case, the size of the apparatus is increased. Furthermore, it is notpossible to perform good imaging while the filter is attached to orremoved from the image sensor.

Method for Extracting Infrared Image Signal

A method for extracting an infrared image signal with the imagingapparatus 100 according to the embodiment is described below.Hereinafter, imaging using visible light is referred to as “imaging inthe RGB mode”, and imaging using visible light and infrared light isreferred to as “imaging in the RGB+IR mode” for the sake of simplicity.

FIGS. 6A to 6F are diagrams for explaining the method for extracting aninfrared image signal. FIG. 6A is a curve illustrating the spectralsensitivities of the image sensor to visible light and infrared light(in FIG. 6A, referred to as “RGB+IR”) which is measured during theapplication of the first voltage V1. FIG. 6B is a curve illustrating thespectral sensitivities of the image sensor to visible light (in FIG. 6B,referred to as “RGB”) which is measured during the application of thesecond voltage V2. FIG. 6C is a curve illustrating spectralsensitivities to infrared light (in FIG. 6C, referred to as “IR”). InFIGS. 6A to 6C, the horizontal axis shows the wavelength of light andthe vertical axis shows sensitivity. As illustrated in FIGS. 6D to 6F,each of the pixels has a Bayer pattern consisting of, for example, oneR-subpixel, two G-subpixels, and one B-subpixel. A pixel array havingthe Bayer pattern can be formed by arranging color filters including adye, a pigment, or the like on the image sensor such that each of thecolor filters has a transmittance corresponding to a specific one of R,G, and B. In other words, the wavelength dependence of a signalgenerated in each of the pixels is determined from the product of thespectral sensitivity curve of the image sensor and the transmittancecurve of the color filter used. Common dye color filters and pigmentcolor filters have a high transmittance in not only the desiredwavelength region but also the infrared region. Specifically, a colorfilter used for R-subpixels has a high transmittance to red (R) andinfrared light, a color filter used for G-subpixels has a hightransmittance to green (G) and infrared light, and a color filter usedfor B-subpixels has a high transmittance to blue (B) and infrared light.Accordingly, in the RGB mode, a signal corresponding to red (R) isgenerated in the R-subpixels, a signal corresponding to green (G) isgenerated in the G-subpixels, and a signal corresponding to blue (B) isgenerated in the B-subpixels as illustrated in FIG. 6E. On the otherhand, in the RGB+IR mode, a signal corresponding to red and infrared(i.e., R+IR) is generated in the R-subpixels, a signal corresponding togreen and infrared (i.e., G+IR) is generated in the G-subpixels, and asignal corresponding to blue and infrared (i.e., B+IR) is generated inthe B-subpixels as illustrated in FIG. 6D.

The image signal obtained in the RGB+IR mode contains signals based onRGB color information that transmits R, G, and B color filters which aresuperimposed on a signal based on infrared light as in the spectralsensitivity curve illustrated in FIG. 6A. Therefore, it is not possibleto directly image only the signal information based on infrared light.However, in the case where the spectral sensitivity curve in the RGB+IRmode has the same shape in the visible region as the spectralsensitivity curve in the RGB mode as illustrated in FIGS. 6A and 6B, itis possible to replicate the RGB component of the image signal acquiredin the RGB+IR mode by multiplying the image signal acquired in the RGBmode by a predetermined gain α. In FIG. 6B, the spectral sensitivitycurve in the RGB mode is denoted by a solid line, and the spectralsensitivity curve magnified by the coefficient α is denoted by a dottedline.

The gain α includes gains α_(R), α_(G), and α_(B). The R-signalgenerated in the R-subpixels in the RGB mode is multiplied by the gainα_(R). The G-signal generated in the G-subpixels in the RGB mode ismultiplied by the gain α_(G). The B-signal generated in the B-subpixelsin the RGB mode is multiplied by the gain α_(B). Consequently, a signalcorresponding to the RGB components of the image signal acquired in theRGB+IR mode is generated from the image signal acquired in the RGB mode.

The signal generated by multiplying the image signal acquired in the RGBmode by the gain α is subtracted from the image signal acquired in theRGB+IR mode to obtain an image signal corresponding to infrared light asillustrated in FIGS. 6C and 6F.

The above processing is performed by the computing circuit 50illustrated in FIG. 1. The computing circuit 50 is included in a signalprocessing circuit that includes a microprocessor, such as a digitalsignal processor (DSP). The computing circuit 50 may alternatively bedisposed in the image sensor that includes the unit pixels 10.

As described above, in each of the plurality of exposure periods, thecomputing circuit 50 (i) acquires a first signal corresponding to animage captured using visible light and infrared light upon a firstvoltage being applied between the pixel electrode 11 and counterelectrode 12; (ii) acquires a second signal corresponding to an imagecaptured using visible light upon a second voltage different from thefirst voltage being applied between the pixel electrode 11 and counterelectrode 12; and (iii) generates a third signal corresponding to animage captured using infrared light by performing a predeterminedcomputation using the first signal and the second signal.

Thus, the imaging apparatus 100 is capable of performing a switchoverbetween imaging using visible light and imaging using visible light andinfrared light simultaneously for all pixels by changing the voltageapplied to the photoelectric conversion layer 15. The imaging apparatus100 is also capable of generating an image captured using infrared lightfrom an image captured in the visible region and an image captured inthe visible and infrared regions. Thus, the imaging apparatus 100 iscapable of capturing an image based on visible light, an image based onvisible light and infrared light, and an image based on infrared lightwithout using a movable filter.

A method for calculating the gain α is described below.

It is possible to remove the RGB color information and extract only theinfrared signal information from the signals generated in each of thepixels in the RGB+IR mode by using ratios η_(R), η_(G), and η_(B), whichare the ratios of the quantum efficiency in the RGB+IR mode to thequantum efficiency in the RGB mode in the R, G, and B wavelengthregions, respectively, of the spectral sensitivity curves illustrated inFIG. 5.

The ratios η_(R), η_(G), and η_(B) are represented by Formulae (1) to(3) below, where Q1 _(R), Q1 _(G), and Q1 _(B) represent the quantumefficiencies in the RGB+IR mode which correspond to red (R), green (G),and blue (B), and Q2 _(R), Q2 _(G), and Q2 _(B) represent the quantumefficiencies in the RGB mode which correspond to red (R), green (G), andblue (B).

α_(R)=η_(R) =Q1_(R) /Q2_(R)  (1)

α_(G)=η_(G) =Q1_(G) /Q2_(G)  (2)

α_(B)=η_(B) =Q1_(B) /Q2_(B)  (3)

Thus, α=Q1/Q2 is satisfied, where Q1 includes Q1 _(R), Q1 _(G), and Q1_(B), and Q2 includes Q2 _(R), Q2 _(G), and Q2 _(B).

In the above description, it is assumed that the exposure time t1 forthe RGB+IR mode be equal to the exposure time t2 for the RGB mode asillustrated in FIG. 7. In the case where the exposure time t1 for theRGB+IR mode is different from the exposure time t2 for the RGB mode asillustrated in FIG. 8, the following computation is performed. FIGS.7(a) and 8(a) each illustrate the timing of the rise or fall of thevertical synchronizing signal Vss. FIGS. 7(b) and 8(b) each illustrate achange in the bias voltage applied to the photoelectric conversion layer15 with time. FIGS. 7(c) and 8(c) each illustrate the timing of thereset and exposure of each row of the pixel array 200.

The amount of signal charge is determined on the basis of the product ofthe sensitivity of the sensor (i.e., the quantum efficiency) and thelength of the exposure time. Accordingly, it is possible to extract aninfrared image by setting the gain α to the values represented byFormulae (4) to (6) below on the basis of the exposure time t1 for theRGB+IR mode and the exposure time t2 for the RGB mode.

α_(R)=η_(R)×(t1/t2)  (4)

α_(G)=η_(G)×(t1/t2)  (5)

α_(B)=η_(B)×(t1/t2)  (6)

Thus, α=(Q1/Q2)×(t1/t2) is satisfied.

While the signals acquired in the RGB mode are multiplied by the gain αin the above-described case, the method is not limited to this; anymethod in which the ratio between the signal acquired in the RGB+IR modeand the signal acquired in the RGB mode can be changed as describedabove may be used. For example, the signal acquired by the RGB+IR modemay be multiplied by a gain. Alternatively, the signal acquired in theRGB+IR mode and the signal acquired in the RGB mode may be multiplied bydifferent gains. Specifically, α1/α2=Q2/Q1 needs to be satisfied, whereα1 represents the gain by which the signal acquired by the RGB+IR modeis multiplied and α2 represents the gain by which the signal acquired inthe RGB mode is multiplied. In the case where the exposure time differsbetween the two modes, α1/α2=(Q2/Q1)×(t2/t1) needs to be satisfied.

Since the gain α is equal to 1 when the exposure time ratio t1/t2 is setto be 1/η, setting the exposure time ratio t1/t2 to be 1/η enables thesubtraction to be performed directly without using an additionalamplifier.

In general, the quantum efficiency for the RGB mode decreases with areduction in the voltage applied to the photoelectric conversion layer.Setting the length of the exposure time t2 for the RGB mode to be longerthan the exposure time t1 for the RGB+IR mode as illustrated in FIG. 8enables acquisition of a sufficient amount of signals and, as a result,increases the accuracy with which the difference in the amount ofsignals generated in the pixels between the RGB+IR mode and the RGB modeis computed.

When the exposure time in the RGB+IR mode is too short to provide asufficient amount of light, the photoelectric conversion layer 15 may beirradiated with light emitted from an additional high-intensity infraredemitter. The light may be either continuous light or pulsed light. Whenthe light is pulsed light synchronized with the RGB+IR-mode exposureperiod, the power consumption of the infrared emitter can be reduced.

The imaging apparatus according to the embodiment is described above.The above-described embodiment does not limit the present disclosure.

The processing units included in the imaging apparatus according to theabove embodiment are implemented typically as an LSI, which is anintegrated circuit. The processing units may be each individuallyintegrated into one chip, or some or all of the processing units mayalternatively be integrated into one ship.

The type of the integrated circuit is not limited to an LSI. Theprocessing units may be implemented as a dedicated circuit or ageneral-purpose processor. It is also possible to use a fieldprogrammable gate array (FPGA), which allows programming after theproduction of the LSI, or a reconfigurable processor, which enablesreconfiguration of connection between circuit cells inside the LSI andsettings of the circuit cells.

An imaging apparatus according to one or more aspects is described aboveon the basis of the embodiment. The present disclosure is not limited bythe embodiment. Various modifications and forms resulting fromcombinations of elements of the different embodiments that may beconceived by those skilled in the art may be included within the scopeof the one or more aspects without departing from the scope of thepresent disclosure.

What is claimed is:
 1. An imaging apparatus comprising: a pixelelectrode; a counter electrode facing the pixel electrode; aphotoelectric conversion layer disposed between the pixel electrode andthe counter electrode, the photoelectric conversion layer convertingfirst light having a first wavelength into a first signal charge upon afirst voltage being applied between the pixel electrode and the counterelectrode, the photoelectric conversion layer converting second lighthaving a second wavelength, the second wavelength being different fromthe first wavelength, into a second signal charge upon a second voltagebeing applied between the pixel electrode and the counter electrode, thesecond voltage being different from the first voltage; and a computingcircuit that acquires a first signal on a basis of the first signalcharge upon the first voltage being applied between the pixel electrodeand the counter electrode, the computing circuit acquiring a secondsignal on a basis of the second signal charge upon the second voltagebeing applied between the pixel electrode and the counter electrode, thecomputing circuit generating a third signal by performing a computationusing the first signal and the second signal.
 2. The imaging apparatusaccording to claim 1, further comprising a voltage supply circuitconfigured to selectively apply the first voltage or the second voltagebetween the pixel electrode and the counter electrode.
 3. The imagingapparatus according to claim 1, wherein the computing circuit generatesthe third signal by subtracting, from the first signal, a signalobtained by multiplying the second signal by a gain.
 4. The imagingapparatus according to claim 3, wherein the gain corresponds to a ratioof a quantum efficiency of the photoelectric conversion layer which isachieved upon the first voltage being applied between the pixelelectrode and the counter electrode to a quantum efficiency of thephotoelectric conversion layer which is achieved upon the second voltagebeing applied between the pixel electrode and the counter electrode. 5.The imaging apparatus according to claim 3, wherein the gain correspondsto a product of a ratio of a quantum efficiency of the photoelectricconversion layer which is achieved upon the first voltage being appliedbetween the pixel electrode and the counter electrode to a quantumefficiency of the photoelectric conversion layer which is achieved uponthe second voltage being applied between the pixel electrode and thecounter electrode, and a ratio of a first exposure time to a secondexposure time, the first exposure time being a time during which thefirst voltage is applied between the pixel electrode and the counterelectrode, the second exposure time being a time during which the secondvoltage is applied between the pixel electrode and the counterelectrode.
 6. The imaging apparatus according to claim 5, wherein thefirst exposure time is shorter than the second exposure time.
 7. Theimaging apparatus according to claim 1, wherein the computing circuitgenerates the third signal by subtracting a fifth signal from a fourthsignal, the fourth signal being obtained by multiplying the first signalby a first gain, the fifth signal being obtained by multiplying thesecond signal by a second gain, the second gain being different from thefirst gain.
 8. The imaging apparatus according to claim 7, wherein aratio of the first gain to the second gain corresponds to a ratio of aquantum efficiency of the photoelectric conversion layer which isachieved upon the first voltage being applied between the pixelelectrode and the counter electrode to a quantum efficiency of thephotoelectric conversion layer which is achieved upon the second voltagebeing applied between the pixel electrode and the counter electrode. 9.The imaging apparatus according to claim 7, wherein a ratio of the firstgain to the second gain corresponds to a product of a ratio of a quantumefficiency of the photoelectric conversion layer which is achieved uponthe first voltage being applied between the pixel electrode and thecounter electrode to a quantum efficiency of the photoelectricconversion layer which is achieved upon the second voltage being appliedbetween the pixel electrode and the counter electrode, and a ratio of afirst exposure time to a second exposure time, the first exposure timebeing a time during which the first voltage is applied between the pixelelectrode and the counter electrode, the second exposure time being atime during which the second voltage is applied between the pixelelectrode and the counter electrode.
 10. The imaging apparatus accordingto claim 9, wherein the first exposure time is shorter than the secondexposure time.
 11. The imaging apparatus according to claim 1, whereinthe photoelectric conversion layer includes photoelectric conversionfilms.
 12. The imaging apparatus according to claim 11, wherein at leastone of the photoelectric conversion films includes an organic material.13. An imaging apparatus comprising: a first photoelectric conversionlayer converting first light having a first wavelength into a firstsignal charge; a second photoelectric conversion layer converting secondlight having a second wavelength, the second wavelength being differentfrom the first wavelength, into a second signal charge; and a computingcircuit that acquires a first signal on a basis of the first signalcharge, the computing circuit acquiring a second signal on a basis ofthe second signal charge, the computing circuit generating a thirdsignal by performing a computation using the first signal and the secondsignal.
 14. The imaging apparatus according to claim 13, furthercomprising: pixels, wherein the first photoelectric conversion layerincludes first photoelectric conversion regions, each of the firstphotoelectric conversion regions corresponding to one of the pixels, thesecond photoelectric conversion layer includes second photoelectricconversion regions, each of the second photoelectric conversion regionscorresponding to one of the pixels, and when viewed from a normaldirection of the first photoelectric conversion layer, one of the secondphotoelectric conversion regions overlaps one of the first photoelectricconversion regions.
 15. The imaging apparatus according to claim 13,wherein the second photoelectric conversion layer is stacked above thefirst photoelectric conversion layer.
 16. The imaging apparatusaccording to claim 13, wherein at least one of the first photoelectricconversion layer and the second photoelectric conversion layer includesa photoelectric conversion film.
 17. The imaging apparatus according toclaim 16, wherein the photoelectric conversion film includes an organicsemiconductor material.